The relative positions of atomic nuclei in molecules are not fixed. A molecular vibration is a periodic distortion of a molecule from its equilibrium geometry. All the various ways the atomic nuclei in a molecule can move relative to each other can be decomposed as combinations of a few vibrational modes. These vibrational modes consist of “bends” and “stretches”, both symmetric and asymmetric. As a molecule vibrates, the energy held in its chemical bonds fluctuates between discrete levels. Thus the energy required for a molecule to vibrate is quantized (comes in discrete quantities), and corresponds by Planck's Law to a specific wavelength of light that can be absorbed by the molecule. These wavelengths are generally in the infrared (IR) region of the electromagnetic spectrum.
By passing a broad-band beam of IR light through a sample of a material and recording the amount of energy absorbed at each wavelength, an absorption spectrum is obtained. The specific set of wavelengths where energy is absorbed identifies the vibrational modes and therefore the chemicals that are present in the material. FTIR spectrometers typically measure the wavelength range of 2.5 microns (micrometers, or millionths of a meter) to 25 microns. Although definitions of the subranges vary, this wavelength range generally corresponds to mid-wave to very long wave IR. When a wavelength is quoted in microns herein, that also includes approximately +/−0.1 microns.
Alternatively, a set of lasers, infrared LEDs or other narrowband light sources (or one or more tunable lasers or other tunable fixed wavelength light sources) providing light at selected specific wavelengths, can be used to obtain an absorption spectrum for only those selected specific wavelengths, to provide discrete wavelength spectroscopy. This would limit the total amount of energy used and absorbed by the material (although increasing energy at those specific wavelengths), which may be important if the material is living tissue.
IR spectra are conventionally described using wavenumber (number of waves per centimeter, the inverse of wavelength), for which the equivalent range is 400 cm−1 to 4000 cm−1 (400 waves per centimeter to 4000 waves per centimeter). When a wavenumber is specified herein, that also includes approximately +/− approximately 10 cm−1. The so-called fingerprint regime is a subrange from about 900 cm−1 to 1800 cm−1 (approximately 5.6 microns to approximately 11.1 microns (micrometers, or millionths of a meter)) that contains the absorption bands for several important biomolecules. For a complex material such as human tissue, composed of several types of biomolecules, the absorption spectrum is like a fingerprint. The goal of this project is to use the absorption spectrum to identify the subtle tissue changes associated with transformation from normal to pre-malignant colonic mucosa.
The absorption spectrum can be measured quickly and easily with FTIR spectrometry or discrete wavelength spectrometry. Attenuated total reflectance (ATR) is a sampling technique for acquiring FTIR spectra that enables variable quantities of a material to be measured without changing the absorption, thereby permitting reproducibility. The sample is pressed into direct contact with a material such as a zinc selenide crystal in which the IR light undergoes total internal reflection. When the light reflects off the internal surface of the crystal, an evanescent wave is formed that satisfies the boundary conditions on the interface. The evanescent wave penetrates approximately 1-2 microns in human tissue. It is absorbed at wavelengths corresponding to the vibrational modes of the sample.
By replacing the ATR crystal with a fiber optic probe, FTIR spectroscopy can be utilized to analyze samples that are remote (distant) with respect to the spectrometer.
However, because the colon is long, providing sufficient IR light at the end of the probe with a broadband IR source may be difficult because of transmission loss over the length of the fiber. Accordingly, a series of lasers, LEDs or other fixed wavelength light sources (or one or more tunable lasers or other tunable fixed wavelength light sources) can be used for illumination, with wave numbers within the diagnostic subranges, or at the diagnostic wavelengths, described below. A single laser cannot be used because its wavelength (wave number) would be very narrow, so that spectra covering all the necessary wavenumbers (or wavelengths) in a subrange (see below) cannot be obtained. However, a series of lasers, infrared LEDs, or other fixed wavelength light sources (each emitting infrared light at a different diagnostic wavelength), or one or more tunable lasers (or other tunable fixed wavelength light sources), can provide the range of wavelengths (wave numbers) necessary for obtaining diagnostic spectra. Unlike conventional infrared spectroscopy, an interferometer is not used to measure the absorption signal. Fourier transformation of the raw measurement is not required with infrared spectrometery utilizing lasers, LEDs or other fixed wavelength light sources.
It is therefore an object of this invention to develop a fiber optic FTIR spectroscopy probe, or discrete wavelength probe, that can be used within the working channel of an endoscope (or other probe) to enable in vivo measurement of the absorption spectrum of colonic mucosa that, combined with spectral analysis algorithms, provides a powerful tool for detection of precancer.